Humidity indicator



Dec. 22, 1953 L. M. ILGENFRIT'Z HUMIDITY INDICATOR Filed Nov. 1e, 195o /NVENTOR A77' ORNE V Patented Dec. 22, 1953 2,663,190 HUMmrriv iNmcA'roR. Y

LesterMb'Illgenfritz, Mamaron'eck, N Y.; assigner to'Bll-f Telephone. Laboratories; Incorporated,

lNeweYorkgN. VY.',:;a corporation of- New York;v v ApnlieanonNqvemher 16, 1950,seria1Nuisance .Tlieobject ofttle Apresent "nyention'ismeans for; measuring #the relative humidity"' Within a container enclosing an electronic i amplier;

A rfeature of 'theinyention is` the 'association sharply .jresonantrover a .narrow bandlofire.` quencies ycharacteristic of the amplifier'.

Anotherl feature ofthe inventionis a reactance devicek associated with the resonant', device to form "a ,combinatiomwhich is' antiresonantg ratra frequency outside`the"characteristic band of fre. quencies... y f

A, further ',f'eature ofi the invention isl a. humid.- ity.sensitiver elementiiassociated With'th'e reactance device to. modify gth'ej damping. ofjthe' antiresonant combination withchanges. in` the. relatiye..humidity1within.the containerenclosingfthe amplier,,.witholut materially. affecting. the' c .ref s119115@ ,offitheLresonant.LdeviceY Within. the: char-V acteristicgband;ofifrequencis.,

Whil'eilthe `iriyeritori,.may..be.uselcl for the def terminationof'ltlie.relatirehumidityjin any con-J4 tainer-enclosing ran Vamp'ligiengdesignedA in ac? cord'ancewiti; the intention,H itlisj 'parti`cularly adapted fi' determining therelative humidity in containersiembodie'd in long cables, ySuch as sub`. marine cables, laidi'in inaccessible locations. HIn recent years, the use of long ablesforthetrans'- mission. of intelligence, suchas telephoneand telegraph cables, hasbeen rapidly increased,"and. has. 'arousedA interest g in ,the" Vpllobl'enr of using cablesofthis generaljcharacter throughterritory, such; as the ocean orotherilarge; body; off water, in Whichithe cable,A is1not`easilygaccessible; `Due *to the 'transmission losses.; in' suchcasea. .the transfV mittedgwaves mustbe amplified at rather. short internals, thus' necessitating the' use .ofgmpliflers spaced fromthirty' to;f`ftyn1iles ,apart alongjthe cables., These 'ampli'ers are encased Withinpan enlargement. .of the cable sheath", and', though, withiskillfuLLdesign. .and good', ',jmaterials,Vv the average!` useful life -of the amplifiers is `satisfactory,`v it is important; to bezahle to test'th'e .electrical .conditions ofieaclr amplifier from'some accessiblespot,"such'as the cable terminals; ,By sucl1"tests," any gradual .deterioration in the. axnpliers maybe measured at suitable'inteizvals; and arrangementsrmadeto. repair, or. replace, va faulty@amlzzliierA in'a season. of. theyear when weather conditions are favorable," materially re, ducing., the cost of the necessary. cableship operations; Y l y y `In;systems of this character; the ,amplifiers are connected-inseries with-the'cable conductorand di'rect-currenomay convenientlybe"Supplied over theecabelv conductor to f theamplie'rs 'fto 4energize thiheatersfof thefampliersi'andfto activateifth'e anodezfcircuitsm. Faults-:Which develop'. in: this-'dis :cectcnrrent :circuiti mayrfbeA 1determinechfbyx 1 the usual direct current, `oitl'ow frequency,.- tests-but other. faults, such a,s.a.s1cwdecrease in. the gain. of anamplier. may. develop,Whichwillaffectlh transmissionof the,lsignals,. even thonghwthedif; rect-current, Y circuitgis .substantially unaiected.,

A; satisfactory system fortesting.s113211x .anllil ersishould notreguire for` its ,E operatio-n, thel transmission vofi anysubstantal amonntwoV nal.. power, tot avoid overloading f, the amlilirs'v should bey compact. stable; anderu'gged; Vand, shouldmot add any, appreciable ,hazardgtm the successful` operation of. vthe system.; A

A system for. f testing., the,v condition of suchl amplifiers .is disclosed,inA my .impendingjoint application Serial No., 133,538, .1ed1Decembei:. 17,v 11949 now patentl 2,580,097; patented DecemeK berf 25J 1951,y by.L.,M.,.IlgenfritZ -and,.R. Ketcnledge.` .The,presenninyention ais a .modifV cationof the, system disclosed inImyx `said appli?. cation to provide means,for;ymeasuringnthe,relafV tive humidity l, in., the` containers. .enclosingfihe amplifiers.

The; varioussections of; Ithe. amplifiers and Iasf, sociated networksv are-,usuallyr assembled in, con?, tainersenclosed .within the. cable; sheath,. Pre?. cautions are taken., toy dry.. all, the electricahelements in-thek containers, ,and vsome.de siccatiye material is usually placed. in, one ofi-'the .conitainers -to absorb any-v moisturethatmayfdevelop after thefdry-inaprccessff; Howevenit is possible thatfthefcontainer. may be 4undulyf exposedcdur; ing the manufacture, or-laying, of thecable; or; that moisture may be produced inl the container fromV themalternate :heating andcooling-ofY Athe enclcsedielementsfinservice; v-byf somemhys" al or; chemical-k action inLA theyelementssofE .the-1am: plier. ,i ory vby 1 permeati-ng; through s on1e -,defec,t in-thefcontainerq- By the present invention-,the relativefhumidtyrsn .thacmtainer 0f:Y ,anyg-de-f sired v,arnilier may be-cleterrlrl.inedy atanytime after-theamplier isiassembled; .Oriduring the laying of. the; cable :or-af 12er lthe; cable 'is-in; service.Y

In thegdrawings;l Figggl ishows a typical fcable system :embodying -thexingyention Fig; ,2' is a schematicicircuitioi thewanipliiiers shown infFig.. 1 and..

Fig; 3 `shows theeffectt ofgchanges, in Athe qliu-fV midity` sensing; elementzom the Vamplifier.'ariill;'

The system shown in Fig. 1 includes-weiche: Way cables connected to. fornita two-Wayzsystem. Signal Waves su1opli'ed=.,toA thei'liybrid:coil` vl are transmitted 1to1 thezmodulator. 2,:.where-the-waves modulatezthe waVesff-rom. a carriervwayeisourceff, andv :a product-=offthis modulation.rsxselectedzby a band-pass filter 4 and supplied/.tattile input circuit of. amplifier.. :.5. Signal. .Wares fromf other sources alsoniay be modulatedawith warm-,from other.- carrier. sources., filtered, ',andfsalsofisupplied tozanlplieiv 5:51 The output: ofA amplierf: 5 isicon:

nected'l by:.transformera-6..gandrcapaciton 1 te the Y l central conductor 8 of the cable, then, through is Conducted through amplifier I4, band-pass iilter I5, amplifier I6 to demodulator I1 where the modulated carrier waves are demodulated with waves from source I8, the signal currents being supplied through hybrid coil I9 to the line.

Signals supplied to the hybrid coil I9 modulate the carrier Waves from source 2| in the modulator 20, and the desired products of modulation are selected by band-pass lter 22 and supplied to amplifier 23. The output of amplifier 23 is supplied through transformer 24 and capacitor 25 to the central conductor 26 of the other cable, thence through this conductor, amplifiers 30, 3I, etc., capacitor 21 and primary winding of transformer 28. The output of transformer 28 is supplied through amplifier 32 and band-pass filter 33 to demodulator 34 where it is demodulated by carrier waves from source 35, the signal currents passing through hybrid coil I to the line.

The vacuum tubes in the amplifiers are preferably of the indirectly heated cathode type, with the heaters connected in series with each other and the central conductor of the cable. The anode circuits of the vacuum tubes are energized by the voltage drop across the heaters of an amplifier. Direct current is supplied to the heaters through .the central conductor of the cable, thus, in a long cable ,having alarge number of heaters in series, the voltage required may be quite high,

and to reduce the maximum voltage required, two

or more sources maybe used.

Current from the grounded source 4I) will flow through the 10W-pass filter formed by the inductors 4I, 43 .and capacitor 42, the central conductor 26, and the heaters of the vacuum tubes in amplifiers 3l, 3D to ground through the low-pass filter formed by the inductors 46, 44 and capacitor 45. Similarly, current will flow from ground through the low-pass filter formed by inductors 54, 56 and capacitor55, the central conductor 8, and the heaters of the vacuum tubes in aniplifiers I0, 9 through the low-pass filter formed by inductors 53, 5I, and capacitor 52 to the grounded source 5U.

When each cable includes a large number of amplifiers and it is necessary to limit the portion of the available frequency range assigned for the crystals, each cable may be individually tested by using a test set for each cable. In cases where the limitations on the portion of the availi able frequency range which may be assigned to the crystals is unimportant, or only a few ampliers are encased in the cable, the far ends of the cables may be connected by a band-pass filter 58, and the test set 59 connected to the near receiving end, thereby conserving testing apparatus and centralizing all the amplifier testing facilities. In this latter arrangement the tests on all amplifiers `in'both directions are made from the one terminal.

The amplifier shown in Fig. 2 is designed to operate over a band width from l2 to 124 kilocycles, but this invention is not in any way limited thereby and may be adapted for use in conjunction with amplifiers operated over other bands of frequencies.

In Fig. 2, the direct current from the center conductor 8 of the cable flows through inductor 63, the heaters of tubes 62, 6I, 60 in series, inductor 64, and the center conductor 8 of the cable to the next amplifier. The inductors 63, 64

with capacitors 65 and 66 reduce the flow of alternating currents in this path. The screen grid and anode of tube 6I) are renergized from this circuit through the interstage network 61, the screen grid and anode of tube 6I are similarly energized through the interstage network 68. The screen grid of tube 62 is connected directly to the heater of this tube, while the anode of tube 62 is energized through the beta network 69 and the primary winding of transformer 10.

The cathodes of tubes 60, 6I, and 62 are respectively biased in the usual manner by the cathode biasing resistors 1I, 13, and 15, respectively shunted by the by-pass capacitors 12, 14, and 16. The carrier waves flow from the central conductor 8 of the cable through the primary winding of transformer SI1-and capacitor 8 I. The secondary winding of transformer is tuned by the shunt capacitor 82' and is connected to the signal grid of tube 66, and through resistors 83 and 1I to the cathode ofthis tube. The output circuit of tube 6I! is connected through the interstage network 61 to the signal grid of tube.l 6I; similarly the output circuitof 'tube 6I is connected through the interstage network 68 to the signal grid of tube 62. The output circuit of tube 62 is connected through the primary winding of transformer 1B, tuned by the shunt capacitor 85, the beta network 69, capacitor 84 and resistor 15 to the cathode of tube 62. The secondary winding of transformer 10 is connected to the central conductor of the cable and through capacitor 86 to the sheath of the cable.

The attenuation of a section of cable between amplifiers may be some 45 decibels larger for vthe high frequencies than for the low frequencies, thus, the response frequency characteristic of the amplifier should be shaped to equalize this variation and produceV a substantially' vfiat response frequency characteristic of the section of cable with the amplifier over the desired band, For convenience of design, the required change may be distributed among the contributionsof the input network, the output networks, and the beta or feedback network.

The amplifier described above has an odd number, three, of stages of amplification, thus, the feedback circuit is connected from the output circuit to the signal grid forming part of the input circuit. If the repeater is designed to have an even number of stages, the feedback circuit may be connected from the output circuit to the cathode of the first tube. Obviously, if desired, a feedback loop may include less than all the stages of amplification in the amplifier.

In so far as the present invention is concerned, no claim of novelty is advanced with respect to the elements of the amplifier described up to the present point. The novelty of the present invention is in the association of the humidity sensing element 90. thek crystal 9I and the capacitor 92, with the other elements of the amplifier- While for completeness of disclosure, a specific type of amplifier has been described, the scope of the invention is not limited thereby as the invention may be embodied in any suitable amplifier having a feedback circuit.

The crystal 9I may be connected to form-a series resonant shunt across the beta circuit, decreasing the feedback over a Very narrow band of frequencies, and thus increasing the gain of the amplifier over the narrow band of frequencies, to form a narrow peak or spike in the mean frequency characteristic of the amplifier. The frequencies of resonance of the crystals are .different by antiresonance of the crystal 9| and the capacitor 92 may be altered considerably by adding dissipation to the antiresonant circuit, without disturbing seriously the phase of the feedback loop or other functions of the amplifier or crystal circuit. This gain dip thus provides a means of measuring some quantity relating to the amplifier such as the moisture content of the atmosphere within the amplifier container.

Dissipation may be introduced in the antiresonant circuit by connecting a resistor sensitive to the humidity of the container,` inA the circuit, preferably, as shown by element 90, in shunt with the capacitor 92. The humidity sensing element 90 may conveniently consist of a polystyrene tube about one-half inch in diameter and one and one-quarter inches long on which are wound two spaced palladium wires, serving as electrodes, and coated with a plastic mixed with lithium chloride, or lithium bromide. Electrically, an element of this character is equivalent to a capacitor of about 10 micromicrofarads, shunting a resistance which may be many megohms in a dry atmosphere, or only a fewhundred ohms in a moist atmosphere. within the amplifier container is very dry, this element will have substantially no effect on the amplifier performance, since its resistance will be many megohms and its capacity effect negligible. At the opposite end of the humidity scale,

the element would tend to form a shunt across the capacitor 92 of undesirably low resistance, thus the element 90 is connected in series with a limiting resistance provided by the resistor 93. `As it is essential to block the direct-current path -from the anode Yof the vacuum tube 62 to the :control grid of the tube 60, a small capacitor 94 is Connected in series with the resistor 93. The

'capacitor 94 also serves to preserve the Inu-beta gain characteristic at very low frequencies, when the resistance of the sensing element is low. The

'l change in mu-beta gain as the resistance of the sensing element changes `from zero to infinity is of the order of .3 decibel or less from 2 kilocycles upward, and about 1 decibel at 1,000 cycles, K

where there is a large phase margin in the feedback loop.

As shown in the upper curve of Fig. 3, the efect of the variation of the resistance of the element 90 on the gain of the amplifier at the spike frequency is not more than about .4 decibel, and for a dry atmosphere is about .l decibel. The effect of the humidity sensitive circuit on the net gain of the amplifier is negligible at all other frequencies. The lower curve of Fig. 3 shows the effect of changes in the resistance of the humidity sensitive element 90, upon the amplifier gain dip (mu-beta gain peak). For high humidities, say above about 15 per cent, the change in the gain dip is nearly 6 decibels. rIhis change approaches zero at about 6 per cent relative humidity. As,

' the amplifier gain dip normally has a magnitude of from 2O to 25 decibels, the change in this magnitude, due to the changes in the element 90, may readily be measured by means of a test oscillator of known output connected across the input to amplier 5, Fig. 1, and the calibrated detector 59. r What is claimed is:

l. The combination with an amplifier having an input and an output circuit, a capacitor con- When the atmosphere nected from the outputv to theinput circuit to form a negative feedback connection, a sharply resonant device connected acrossvsai'd input circuit and forming with 'said capacitor an antiresonant combination, a resistor, sensitive to the humidity of its surroundings, connected in said antiresonant combination to vary the damping of said combination with variations in said humidity and means connected to said amplifier to measure the variations in amplification of said amplifier with variations in said resistor.

2. The combination in claim 1 in which said sharply resonant device is a vibrating crystal having a natural frequency 'of resonance.

3. The combination in claim 2 in which said capacitor and crystal are antiresonant at a frequency higher than the resonant frequency of the crystal.

4. The combination in claim 1 in which said resistor is connected in parallel relationship with said capacitor. Y

5. The combination in claim 1 with a second resistor and a second capacitor connected in serial relationship with said resistor across said capacitor.

. 6. In combination with a cable connecting in serial relationship a plurality of amplifiers, each amplifier having an input circuit and an output circuit, a capacitor individual to each amplifier connected from the output circuit to the input circuit to form a negative feedback connection, a sharply resonant device individual to each amplifier connected across the input circuit to form with said capacitor 'a combination antiresonant at a frequency uniquely characterizing the amplifier, a resistor, sensitive to the humidity of its surroundings, connected in each antiresonant'combination to vary the damping of the combination with variations in the humidity and measuring means connected to said cable to Ameasure the variations in amplification of said amplifiers with variations in said resistors.

7. The combination in claim 6 in which the Isharply resonant device is a crystal having a natural frequency of resonance uniquely characteristic `of the amplifier.

3.v The combination in claim 6 in which said resistor is connected in parallel relationship with said capacitor.

9. The combination in claim 6 with a second resistor and a second capacitor connected in serial relationship with said resistor across said capacitor.

10. The combination in claim 6 in which said measuring means includes an oscillator and a heterodyne frequency analyzer connected to said cable and tunable to the characteristic frequencies of the amplifiers.

LESTER M. ILGENFRITZ.

References Cited in the file of this patent UNITED STATES PATENTS 

